High-frequency heating is classified into induction heating and dielectric heating according to the physical properties of an object to be heated. The former is chiefly used to heat a conductive metal, and the later is chiefly used to heat a material having dielectric loss, i.e., water, paper, plastic, etc.
Furthermore, high-frequency heating is classified according to the frequency of a heating power source, as shown in Table 1 below. In particular, the frequencies used for high-frequency induction heating may be subdivided into low frequencies (operating frequencies of 50 to 60 Hz), intermediate frequencies (100 to 10 KHz), high frequencies (10 to 500 KHz), and radio frequencies (100 to 500 KHz). In particular, heating using intermediate frequencies, high frequencies, and radio frequencies is referred to as high-frequency heating.
TABLE 1Commu-nication-FrequencyWavelengthFrequency relatedIndustrial(Hz)(m)namepurposepurpose30K to 300K104 to 103LFinter-general high-mediate radiofrequency300K to 3M10 to 10MFwavebroad-inductioncastingheating3M to 30M10 to 10HFshort short highwavewavefrequencybroad-inductioncastingheatingwirelesscommu-nication30M to 300M10 to 1 VHFultra-TVshort broad-wavecasting300M to 3 G  1 to 0.1UHFmicro-TVmicro-3 G to 30 G 0.1 to 0.01SHD wavebroad-wave30 G to 300 G 0.01 to 0.001EHFcastingheatingcommu-nicationradar
Meanwhile, all materials may be basically classified with conductivity and dielectricity. In connection with conductivity, when electricity is applied to a material, the electricity does not stay at a given location and flows to a lower voltage location. A material has resistance to the flow of electricity. According to the degree of resistance, a material having low resistance is called a conductive material or a conductor, whereas a material having high resistance is called a nonconductor.
Furthermore, the values of dielectricity are represented by dielectric constants, unlike those of resistance. However, when the values of dielectricity are represented by specific resistance values, a material used as a dielectric is a nonconductor having a high specific resistance value. Accordingly, all materials may be classified with conductivity and dielectricity.
Next, when the materials classified as described above are paired with high-frequency heating targets, conductors can be heated through induction heating, and dielectrics can be heated through dielectric heating using dipole vibration, with the result that all the materials can be targets of high-frequency heating.
Furthermore, induction heating is subdivided into heating using hysteresis loss and heating using an eddy current according to their heating principle. Of these materials, magnetic materials can be heated through both the heating methods using the above two types of loss, respectively. Unfortunately, nonmagnetic materials cannot be heated through heating using hysteresis loss, and thus have low heating efficiency. However, nonmagnetic materials may be used as the materials of containers containing objects to be heated during high-frequency heating by utilizing the above-described disadvantageous point. High-frequency heating has the following advantages over other methods, such as electrical heating and fuel heating, and thus has rapidly extended to a wide variety of fields recently.
The first advantage of high-frequency heating is outstanding economic feasibility.
In other words, in induction heating, an object to be heated is directly heated by itself, and thus high efficiency is achieved. Accordingly, although equipment manufacturing cost is expensive, total production cost can be reduced to half or less of that in other fuel devices.
Second, high quality can be ensured. In other words, if an appropriate frequency is selected according to the material and size of an object to be heated, uniform temperature, speed, etc. can be controlled as desired, and thus individual parts can be produced through mass production (in particular, the enablement of selective heating, such as local heating or surface quenching, is an essential condition for economic feasibility and heat treatment technology).
Third, non-contact heating can completely isolate and block an object to be heated from a heating source, and thus can prevent various types of contamination. Non-contact heating can be applied to the process of manufacturing silicon for semiconductor wafers (this method enables heating at an ultra-high temperature equal to or higher than 3000° C. and heating in various types of gas atmospheres or in a vacuum state, and thus is recently used in the fields of application of state-of-art technology).
Fourth, work can be rapidly done on a second basis. In other words, most of high-frequency heating for heat treatment, metal bonding, drying, etc. can be performed on a second basis. Such rapid performance enables the introduction of a tack system which can synchronize production speed with previous and next processes (pressing and heat treatment) when the prevention of a change in material and the mass production of a part, such as an automation component, are required. This is a great advantage which is unthinkable in the case of heating using other fuel devices.
Fifth, the use of a material can be reduced. In other words, when this method is used for tinning, tin is instantaneously melt and comes into tight and uniform contact with a surface of a steel plate. Accordingly, desirable coating can be achieved even using ⅓ of the amount of tin required for a conventional method of performing dipping in a tin melt.
As related conventional technologies, Korean Utility Model Application Publication No. 20-2000-0003481, Korean Patent Application Publication No. 10-1987-0008601, and Korean Utility Model Application Publication No. 20-2000-0018306 are disclosed.
However, so far, there has been no case where the high-frequency heating apparatus is applied to the field of progressive dies. Accordingly, conventionally, during the formation of products, cracks occur due to a component of a material, a temperature condition (winter) during work, and an increase in brittleness attributable to the cooling of a material. Furthermore, problems arise in that a defective rate increases and productivity decreases due to changes in the dimensions of products.
Furthermore, products must be formed by taking into account the differences in the elongation of materials based on the shapes of the products, and thus the enlargement of dies is required due to an increase in the number of processes (first forming, second forming, and third forming). Furthermore, in the bending of high tension steel plates, there is difficulty in forming products due to excessive spring back.